1. Field of the Invention
The present invention relates to a method of real-time estimation of indicators of the combustion state of an internal-combustion engine equipped with one or more detectors.
2. Description of the Prior Art
Emissions reduction standards have led to a progressive adaptation of engine technologies such as the introduction of additional actuators (exhaust gas recirculation valve, direct electronic injection, variable-geometry turbine) and new combustion modes (homogeneous diesel combustion or stratified gasoline combustion). In this context, the engine has become an assembly of complex systems requiring management by means of modern automation techniques. The engine control function consists in managing the available actuators so as to guarantee engine performances while meeting environmental constraints. A fundamental aspect of engine control lies in the combustion control system via precise management of the thermodynamic conditions in the cylinder (temperature, boost pressure, composition of the mixture admitted) and of the adjustment of the injection parameters.
The new combustion modes, such as homogeneous diesel combustion, are much less polluting than conventional combustions (gasoline or diesel), but they are, on the other hand, much more sensitive to working conditions. They therefore involve an additional investment in terms of regulation. Loop combustion control is possible only if information is available on the combustion progress and if it can be influenced through the agency of the actuators available. This information (indicators) can be obtained from direct measurement of the pressure in the combustion chamber by means of a specific detector. This method allows direct access of physical indicators of the combustion but, because of the life and cost of this type of detectors, they cannot be standard at this time in vehicles. To date, it is more realistic to turn towards alternative techniques such as the estimation of combustion state indicators from non-intrusive measurements. A synthesis of the existing methods is presented below:
Ionization current interpretation: The principle measures the ionization current generated during combustion. During this combustion, some chemical reactions cause release of ions. Their production is influenced by the temperature and pressure conditions in the cylinder. To detect them, the plug is permanently supplied with a low-amplitude voltage. The ionization current contains information on the successive combustion stages: ignition, flame front propagation and a stage associated with the pressure and temperature variations at the end of the reaction. Eriksson and Nielsen have shown in the document mentioned below, for example, that the maximum pressure during a cycle and its argument can be identified by interpreting the ionization current:    L. Eriksson, Spark Advance Modeling and Control, Ph.D. thesis, Linköping University, Sweden, 1999.
Analysis of the instantaneous rotating speed of the crankshaft: The speed is measured by means of an encoder mounted at one end of the crankshaft. The cylinder pressure is reconstructed by inversion of the kinematic chain of the engine, the model thereof being of course known. The following document can be referred to for example:    S. J. Citron, J. E. O'Higgins & L. Y. Chen, Cylinder by Cylinder Engine Pressure and Pressure Torque Waveform Determination Utilizing Speed Fluctuations, SAE Paper (1989), no. 890486.
Reconstruction from engine vibration measurements: The vibrations of an internal-combustion engine can be measured by means of a detector referred to as accelerometer or of a knock detector mounted on the engine. It can be noted that an accelerometer is a detector that, in a given reference system, measures the acceleration of the body to which it is fastened (or one of its components) and allows the resulting impacts and vibrations to be studied. Extraction of coherent combustion state indicators from an accelerometer is difficult because of the nature of the signal delivered. In fact, this signal does not only contain information on the combustion because the vibratory response of the engine is also due to other events, such as injection nozzle chattering, piston return stroke to the top dead center, vibrations induced by the distribution system, etc.
Using accelerometer type detectors to estimate the cylinder pressure is a known technique. The following documents, wherein the cylinder pressure is to be estimated from the signal delivered by the accelerometer, can be mentioned for example:    Y. Gao & R. B. Randall, Reconstruction of Diesel Engine-Cylinder Pressure Using a Time Domain Smoothing Technique, Mechanical Systems and Signal Processing 13 (1999), no. 5, 709-722.    H. Du, L. Zhang & X. Shi, Reconstructing Cylinder Pressure from Vibration Signals Based on Radial Basis Function Networks, Proceedings of the Institution of Mechanical Engineers, Part D 215 (2001), 761-767.    R. Johnsson, Cylinder Pressure Reconstruction Based on Complex Radial Basis Function Networks from Vibration and Speed Signals. Mechanical Systems and Signal Processing, 2006.
Du et al. deal with the cylinder pressure estimation problem by means of signal processing techniques involving deconvolution or inverse filtering. They have shown the capacity of a neural network to carry out an approximation of the non-linear transfer function between the evolution of the cylinder pressure and the vibration signal generated. Johnsson uses a similar method according to which the signal delivered by the accelerometer is combined with the engine speed signal to reconstruct the pressure signal and the combustion state indicators by means of a neural network whose inputs are imaginary variables.
Other methods using accelerometric detectors for controlling the combustion of an engine are also known:
EP Patent 1,116,946 describes a method and a system for controlling the combustion from signals delivered by an accelerometer, according to which the signal measured is windowed (combustion occurrence period). This signal is then processed during each control loop and it is compared with a reference signal in order to determine the changes to be brought to the combustion state indicators. Processing the signal from the accelerometer comprises three main stages: rectification, filtering and integration.
U.S. Patent Published application 2,004,267,430 (WO-05,001,263) describes a method of processing accelerometric signals resulting from the vibrations of an internal-combustion engine. In particular, the signals are filtered by spectral filters and the combustion analysis curve is reconstructed by deconvolution of a transfer function identified from an experimental database. The result is an estimation of the combustion state indicators allowing combustion control. For example, the parameter referred to as SoC (Start of Combustion) is estimated from a polynomial depending on the energy release, its maximum angle and the boost pressure.
German Patent 19,536,110 (FR-2,739,414) describes a method of processing accelerometric signals resulting from vibrations for controlling the combustion of a diesel engine. In particular, the signals are filtered in two different frequency bands. The first frequency band [10 kHz, 30 kHz] allows extraction of components associated with the injection by means of a thresholding device. The second frequency band (0.5 kHz, 4 kHz) allows extraction of the signal components generated by the combustion using an identical thresholding method.
U.S. Pat. No. 6,546,328 describes a method using a wavelet transform allowing a priori to locate combustion state indicators like the method provided.
French Patent 2,834,789 describes a knock signal processing method. The method adjusts the acquisition sampling frequency to the engine speed. A Fourier transform is calculated for the various sampling value groups for a frequency of interest. The information on combustion results from the summation of the Fourier transform results.
Fourier transforms are widely used in many fields, including processing of accelerometric data associated with engine diagnosis and control. Examples in the recent literature are U.S. Pat. No. 6,388,444 describing the use of such tools to detect combustion misfires, U.S. Pat. No. 6,456,927 or U.S. Published Patent Application US-2004/0,162,668 describing the use of Fourier transforms for engine knock detection. These patents use windowing of the accelerometric signal, band pass filtering (U.S. Published Patent Application 2006/0,085,119 for example). Such a use for the purpose of diesel or gasoline engine control is also known from U.S. Pat. No. 6,976,936 for example.
After preprocessing, the Fourier transform can be used in form of a fast Fourier transform (FFT), a fast known algorithm, calculated on all of the samples, from which indicators are obtained by means of a calculation involving the amplitude or the energy of components of the Fourier spectrum thus calculated, and sometimes an amplitude or energy integration (U.S. Published Patent Application 2006/0,085,119).
Another method, described in U.S. Published Patent Application 2004/0,267,430, uses a variant of the Fourier transform, the spectral decomposition (involving several FFTs) to extract a parameter, referred to as SoC (Start of Combustion), after integration of a filtered accelerometric signal. The latter first calculates a mean on different acquired cycles. Another example of use of spectral decomposition for reconstructing a pressure signal from an accelerometric signal is given in U.S. Pat. No. 6,408,819.
The indicators from the aforementioned approaches cannot be used directly for applications linked with internal-combustion engine diagnosis and control. These methods are based on a temporal integration of a signal delivered by an accelerometer. Processing of the signal from the accelerometer is not carried out in real time. Furthermore, these methods greatly depend on the nature of the combustion and/or on the technology of the detector used. Their field of application is therefore limited.
The method according to the invention allows real-time estimation of the indicators of the combustion progress of an internal-combustion engine equipped with one or more detectors, whatever the detector technology used and whatever the nature of the combustion.